1. Technical Field
The present invention relates to a jitter buffer controller that can be adapted to a voice packet communication system, such as voice communications or VoIP (Voice over IP) using IP (Internet Protocol) network, in which voice is transformed into packets to be transmitted.
2. Background Art
In recent years, along with the rapid popularization of the Internet, VoIP communications has been receiving attention. In the VoIP communications, voice data is transformed into IP packets to be transmitted over an IP network. The IP network renders best-effort service, which means that a transmission bandwidth for voice transmission is not always secured contrary to the case of using a fixed telephone. Accordingly, there may occur jitters in packet arrival times due to network congestions and the like. Delays in the arrivals of voice packets due to the jitters cause discontinuity in voice reproduction on a receiver's side which has run out packets to be reproduced, resulting in deterioration of voice quality in communications. In the VoIP communications, a jitter buffer (hereinafter, referred to as “buffer”) is provided on the receiver's side, which is shown in FIG. 12. The buffer once accumulates voice packets inputted at irregular intervals, and outputs the accumulated voice packets at regular intervals.
Packets are accumulated in the buffer in advance of reproduction, so packet reproduction can be continuously performed even when packets arrive with delay due to jitters, by reproducing the accumulated packets during the delay, to absorb the jitters to thereby attain stability in reproduction. It is possible to absorb jitters to a greater degree by increasing the number of packets to be accumulated in the buffer. However, as the number of packets to be accumulated increases, a longer delay also develops in transmitting the received packets, which impairs realtimeness in the communications.
As described above, if the number of the packets to be accumulated in the buffer is too small, it leads to deterioration of voice quality in communications, such as discontinuity in voice reproduction. The trade-off of increasing the number of packets to be accumulated is correspondingly longer delays to be developed in transmission. Therefore, it is necessary to optimize the number of packets to be accumulated in the buffer in accordance with jitters in the network such that the minimum number of packets for retaining voice quality in communications is accumulated, to thereby minimize the transmission delay resulting from packet accumulation in the buffer.
The Patent document 1 discloses a technique for minimizing the transmission delay by dynamically setting the number of packets to be accumulated in a buffer in accordance with jitters, with consideration given to influence on voice quality in communications.
FIG. 13 shows a structural diagram of the technique described in Patent document 1. Arrived packets are received to be sent to a jitter buffer and to a jitter measuring portion. The jitter measuring portion measures jitters in the arrived packets, and the jitter thus measured are sent to a reference value change determining portion. The reference value change determining portion receives current information on the accumulation capacity of the buffer from an accumulation capacity adjusting portion, and compares the information with the jitters measured by the jitter measuring portion. The reference value change determining portion determines a new accumulation capacity of the buffer based on the result of comparison, and notifies the accumulation capacity adjusting portion of the new accumulation capacity. On the other hand, a silence determining portion receives reproduced packets from the jitter buffer, determines whether the packets are silent or not, and notifies the accumulation capacity adjusting portion of the result of judgment. The accumulation capacity adjusting portion replicates or discards the silent packets based on the new capacity notified by the reference value change determining portion. The accumulation capacity adjusting portion also outputs voice signals to a reproduction device.
According to the prior art disclosed in Patent document 1, the jitters in the arrived packets are measured, based on which the accumulation capacity large enough to absorb the measured jitters is set for the buffer, and silent packets in the buffer are replicated or discarded so that the buffer accumulates packets at the set accumulation capacity, to thereby dynamically set the buffer accumulation capacity.
Patent document 2 discloses another technique for setting optimal buffer accumulation capacity for minimizing transmission delay while avoiding buffer underflow.
FIG. 14 is a structural diagram of the technique described in Patent document 2. Arrived packets are received to be sent to a jitter buffer and to a jitter measuring portion. The jitter measuring portion measures jitters in the arrived packets, and sends the result of measurement to a statistical processing portion. The statistical processing portion determines, based on a distribution of jitters, an accumulation capacity capable of absorbing the jitters, and notifies a reproduction control portion of the accumulation capacity. The reproduction control portion outputs a voice signal to a reproduction device.
According to the prior art disclosed in Patent document 2, jitters in the arrived packets are measured, and the statistical processing is performed thereon. Based on the distribution of the jitters obtained by the statistical processing, a maximum value of the jitters, with which buffer underflow is surely avoided according to the statistics, is determined, and an accumulation capacity capable of absorbing a jitter of the maximum value is set for the buffer.
[Patent document 1] JP 2001-160826 A
[Patent document 2] JP 2003-264583 A